The CDC said that "genetic fingerprinting" matched the bacterium in her blood to the strain that sickened eight students with connections to Princeton last year.
From the beginning, authorities suspected meningococcal disease, a bacterial infection that can quickly become deadly. Symptoms include sudden onset of high fever, headache, stiff neck, vomiting, nausea, rapid breathing, or a rash. It is spread by "close, prolonged, or face-to-face" contact including kissing and sharing cups and smoking materials, the CDC said.
The infection, which kills about 10 percent of victims, can take two forms, said Caroline Johnson, director of the Philadelphia Department of Health's division of disease control. In the most widely known form - meningitis - the bacteria infect the lining of the brain and spinal cord. In the other, they infect the blood. Ross had a blood infection.
From March through November 2013, there were eight "Princeton-related" cases of meningococcal disease, Princeton spokesman Martin Mbugua said. None were fatal. One patient was a visiting student.
According to the CDC, Pennsylvania and New Jersey require that middle-school students be vaccinated against meningococcal disease. The approved vaccine, which Ross had had, protects people from the A, C, Y, and W-135 serotypes, but there are others, Johnson said. Because of the vaccine, serotype B is now the strain that most commonly causes disease. It has been challenging to create an effective vaccine for it.
The bacterium that has circulated at Princeton is a form of serotype B.
Princeton students late last year were offered an investigational meningitis vaccine for serotype B, and they embraced it.
Mbugua said 86 percent of the nearly 5,800 students recommended to get the vaccine - a combination of all undergraduates and some graduate students - have received two doses of the investigational vaccine; 97 percent of undergrads have gotten one dose. Students will have another opportunity to get either the first or second dose next week on Wednesday and Thursday.
The CDC said that most young people who get two doses of the investigational vaccine should be protected against meningitis. However, they may still have the bacterium in their throats and be able to spread it to others.
Drexel said that it and the Philadelphia Department of Public Health had identified people who had close contact with Ross and given them antibiotics to protect against infection. Johnson said up to 500 people at Drexel were treated.
No other meningitis cases have been reported at Drexel. Princeton has had no new cases since vaccination began Dec. 9.
The CDC said people at Drexel were not now considered to be at higher risk.
The CDC said it does not recommend limiting social interactions with students but asked students at both schools to be vigilant about symptoms.